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10 Years of SOHO

On 2 December 1995 the joint ESA-NASA Solar Heliospheric Observatory (SOHO) was launched by an Atlas II-AS rocket from Cape Canaveral in Florida, US.

SOHO was designed to answer the following three fundamental scientific questions about the Sun:

What is the structure and dynamics of the solar interior?

Why does the solar corona exist and how is it heated to the extremely high temperature of about 1 000 000 °C?

Where is the solar wind produced and how is it accelerated?

Clues on the solar interior come from studying seismic waves that are produced in the turbulent outer shell of the Sun and which appear as ripples on its surface.

Orbit

To obtain a 24 hours a day, 365 days a year view of the Sun SOHO is placed at a permanent vantage point 1.5 million kilometers sunward of the Earth in a halo orbit around the L1 Lagrangian point. SOHO was initially designed to observe the Sun continuously for at least two years. All previous solar observatories have orbited the Earth, from where their observations were periodically interrupted as our planet eclipsed the Sun. The advantage of SOHO has been its continual monitoring of the Sun throughout the current solar cycle.

Spacecraft

SOHO is made up of two modules. The Service Module forms the lower portion of the spacecraft and provides power, thermal control, pointing and telecommunications for the whole spacecraft and support for the solar panels. The Payload Module sits above it and houses all the scientific instruments.

Dimensions

Height, breadth, width

4.3 x 2.7 x 3.65 m

Width with solar array deployed

9.5 m

Mass

Total at launch

1850 kg

Payload

610 kg

Telemetry

During real-time operation

200 Kbits/s

during on-board storage mode

40 Kbits/s

Operations

Control of the spacecraft was lost in June 1998, and only restored three months later through efforts of the SOHO recovery team. All 12 instruments were still us-able, most with no ill effects. Two of the three on-board gyroscopes failed immediately and a third in December 1998. After that, new on-board software that no longer relies on gyroscopes was installed in February 1999. It allowed the spacecraft to return to full scientific operations, while providing an even greater margin of safety for spacecraft operations. This made SOHO the first three-axis stabilised spacecraft operated without gyroscopes, breaking new ground for future spacecraft designs.

Instruments

The scientific payload of SOHO comprises 12 complementary instruments, developed and furnished by 12 international consortia involving 29 institutes from 15 countries. Nine consortia are led by European scientists, the remaining three by US scientists. More than 1500 scientists in countries all around the world are either directly involved in SOHO's instruments or have used SOHO data in their research programmes.

The solar interior

GOLF and VIRGO both perform long and uninterrupted series of oscillations measurements of the full solar disk, respectively in velocity and in the irradiance domain. In this way, information is obtained about the solar nucleus. SOI/MDI measure oscillations on the surface of the Sun with high angular resolution. This permits us to obtain precise information about the Sun's convection zone - the outer layer of the solar interior.

The solar atmosphere

SUMER, CDS, EIT, UVCS, and LASCO constitute a combination of telescopes, spectrometers and coronagraphs that observe the hot atmosphere of the Sun, the corona, extending far above the visible surface. SUMER, CDS and EIT observe the inner corona. UVCS and LASCO observe both inner and outer corona. They obtain measurements of the temperature, density, composition and velocity in the corona, and follow the evolution of the structures with high resolution.

The solar wind

CELIAS, COSTEP and ERNE analyze in situ the charge state and isotopic composition of ions in the solar wind, and the charge and isotopic composition of energetic particles generated by the Sun. SWAN make maps of the hydrogen density in the heliosphere from ten solar diameters. It uses telescopes sensitive to a particular wavelength of hydrogen, allowing the large-scale structure of the solar wind streams to be measured.

Science

SOHO has provided an unprecedented breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, to the solar wind and its interaction with the interstellar medium. These findings have been documented in an impressive, still growing body of scientific and popular literature.

Some of the key results include:

Revealing the first images ever of a star’s convection zone (its turbulent outer shell) and of the structure of sunspots below the surface

Providing the most detailed and precise measurements of the temperature structure, the interior rotation, and gas flows in the solar interior

Measuring the acceleration of the slow and fast solar wind

Identifying the source regions and acceleration mechanism of the fast solar wind in the magnetically "open" regions at the Sun's poles

Discovering new dynamic solar phenomena such as coronal waves and solar tornadoes

Revolutionising our ability to forecast space weather, by giving up to three days notice of Earth-directed disturbances, and playing a lead role in the early warning system for space weather

Monitoring the total solar irradiance (the ‘solar constant’) as well as variations in the extreme ultra violet flux, both of which are important to understand the impact of solar variability on Earth’s climate

Besides watching the Sun, SOHO has become the most prolific discoverer of comets in astronomical history: as of November 2005, more than 1000 comets had been found by SOHO.

114 000 000 000 000 bytes (114 Terabytes) of web pages/data have been served over the internet when the Stanford MDI archive is included. The total amount of data given to users is quite a bit higher when offline requests are accounted for.